The upside of stumbling through my most recent furnace build is that I've had plenty of time for random thought generation. Here's the current thought/question. Would a "venturi chimney" help a furnace heat up faster? Perhaps, even remove need for blower? In general, seems like adding a chimney to a furnace could help decrease heat up time by allowing more combusted fuel to pass through furnace per unit time than would be possible without. The stack effect is the reason. Right? Well, seems reasonable to assume that adding a chimney with a choke point in its middle (measured vertically) could move air even faster. Basically, stack effect + venturi effect = even faster air flow through furnace and, therefore, more combustion per unit time. This might not be terribly useful if one is using a blower. However, I'm wondering if a venturi chimney could possibly allow one to reach iron melting temps in a furnace without using a blower. Anyone ever try this?
I'm not 100% on my fluid dynamics, but if I am understanding your description correctly the narrowing would have little to no net effect. Sure the gas coming out the chimney would have to travel faster through the restriction, but it would then slow down after the restriction, making any net effect to likely be slowing the overall volume due to the restriction. If you have a stream of air injected into the restriction like a typical venturi pump I could see that speeding your flow some, but in that case you're only moving where your blower is, from inlet to outlet. I have used negative pressure in other setups not related to casting and it can have it's advantages keeping the internal volume at slightly reduced pressures. My past use of this was to reduce air leaking into the surrounding environment, but no idea what effect that would have here though.
@dtsh, my thought was that a Venturi chimney would work for the same reasons that a Venturi burner works. Main difference is, of course, chimney is output while burner is input. In my completely under-informed opinion, diameter of chimney is typically greater than air input for Venturi burner. Given this, might get more airflow with chimney approach. If it’s not too big of a pain, might try it out to see what happens. On the flip side ... There likely is a 99.999 percent chance that smarter folks than me have considered this approach and chose not to use it for good reasons.
Unless its enhancing the combustion I cant really see any advantage to moving the heat up and out of the furnace faster. I'm actually experimenting with reducing my exhaust hole to keep more heat in without adversely affecting my combustion. Pete
Moving more air through the furnace is a good thing only if two conditions are satisfied, 1: the extra air is used to completely combust extra fuel and 2: the size and design of the furnace is such that the furnace and its contents are capable of extracting the additional combustion heat. IWO, the resulting flame should be appropriately sized for the furnace, too big a flame would be wasteful.
I would think that what would work better is a combustion blower capable of significant pressure, 5 to 7 psi and restricting the exhaust to get the pressure in the furnace that high. Essentially providing more oxygen inside the furnace to combust more fuel. If you look at high efficiency water heaters the exhaust temperature is only a few degrees above the water temperature. An ideal melting furnace would have an exhaust temperature only a little hotter than the melting temperature of the metal. Currently that is called an electric furnace!
Enhancing combustion would be the goal. Would be fantastic if I could get to and maintain iron melting temps without need for blower. Instead of pushing more air, suck more air. However, there some good comments that suggest that increasing velocity of air through furnace this way might prove to be counter-productive.
I am unclear as to the general plan for the Venturi. Could you outline the general construction of the Venturi? I agree in general that rapid movement of air through the furnace leads to a “cold” furnace. Denis
Denis' use of a stack for the purposes of minimizing noise was a surprise to me. I would have never thought that would work. That just goes to show how little I know.lol. But I generally thought the use of a stack would be limited to increasing airflow through the system for both combustion enhancement (particularly if you're drawing air through a solid-fuelbed or something) as well as removing combustion products. But, although I dont think it is of much benefit in our typical foundry furnaces, increasing airflow speed though a furnace could be a benefit if the airflow is actually doing something dynamic as opposed to passively "just passin' through". A reverberatory furnace is a good example. Google images of reverbs should demonstate what I mean. I think the domed lids some guys are building these days may be another example. Pete
The chimney I use actually inhibits flow more than its draft augments flow. If the furnace were purely naturally aspirated and caused to be at a negative pressure by draft from an added chimney, then a chimney might be a plus just like it is in a home hearth, chiminea, or similar situation. But almost all melting furnaces are pressurized by a blower. Adding the 8’ chimney actually resists outflow as evidenced by flame tongues that will form at its base should the lid/chimney seal be incomplete. Unless positively needed, chimneys should be avoided as they add considerable complexity to the build and both thermally and weight-wise stress the lid. The chimney sections have to be hauled out to the furnace each firing and after firing allowed to cool some and then detached and removed and stored. They also prevent preheating metal on the vent. I do like the quieter operation, but would dispense with the lid tomorrow if fussy neighbors weren’t an issue. Denis
@Melterskelter, the venturi would be 6 to 8 inch vertical chimney that narrows by 50% about halfway up and then flares back out to full diameter at top. Think hour glass. So, if full diameter of chimney is 6 inches, a venturi chimney would narrow down to 3 inches in center. After reading comments and thinking about idea some more, I'm doubtful that it'd work as I'd envisioned. For one thing, without a blower, there wouldn't necessarily be a constant volume of air flowing through chimney. Net effect would probably be same as reducing vent hole diameter. Still, having a short chimney at top of furnace would make it easier to tweak diameter opening, but that's a different topic.
It's also possible to vent out of the side instead of the lid, in that way, a chimney wouldn't interfere with the operation of the lid, there were a few example of that over on Alloy Avenue, I don't know that there are any here.
